This is a term is applied to a climb which has been described
as being easier than it actually is. Deciding the difficulty of a climb is a
moderately subjective process. The first person to climb a route compares
it with other routes that they have done and assigns a difficulty, this is denoted by the "grade" of the climb. Over
time, if many people climb the same route, a level of difficulty is
agreed on through consensus. Sometimes the original grade is decreased
(downgrading), sometimes the climb is upgraded. The latter ofter
happens when a piece of rock falls of the climb making it harder by
decreasing the number of handholds on the rock.

From country to country the base level can vary slightly. For example in
Thailand grades are generally considered to be a bit 'soft'. The climbs
are not quite as difficult as they are described. Sometimes the grades
in an area are a bit harder than in other areas. If you go to such an
area without knowing about the grade offset you can get 'sandbagged'.
You try grades that you normally can climb with ease. You can't get up
them. It's like you have a bag of sand tied to your waist!

Another common way that a route becomes sandbagged is when an moderately
easy route is first done by a very good climber. They are used to doing
things that are really hard. It wasn't that difficult to them, and hey
their girlfriend could probably climb it, so they give it a pretty low
grade. Actually, they were just climbing really well and it's a bitch
hard. Hey presto, instant sandbag!